Anomalous orbital structure in a spinel-perovskite interface $\gamma$-Al$_2$O$_3$/SrTiO$_3$

TL;DR

This study reveals an unusual out-of-plane orbital occupation in a spinel-perovskite interface, contrasting with typical in-plane occupation in similar heterostructures, and links it to altered crystal fields and symmetry breaking.

Contribution

It demonstrates the existence of anomalous out-of-plane orbital occupation in $ ext{γ}$-Al$_2$O$_3$/SrTiO$_3$ heterostructures, challenging previous assumptions about orbital preferences.

Findings

Out-of-plane $d_{xz}/d_{yz}$ orbital occupation observed

Strain influences the orbital structure

Altered crystal field and symmetry breaking are key factors

Abstract

In all archetypical reported (001)-oriented perovskite heterostructures, it has been deduced that the preferential occupation of two-dimensional electron gases is in-plane $d_\textrm{xy}$ state. In sharp contrast to this, the investigated electronic structure of a spinel-perovskite heterostructure $\gamma$-Al$_2$O$_3$/SrTiO$_3$ by resonant soft X-ray linear dichroism, demonstrates that the preferential occupation is out-of-plane $d_\textrm{xz}$/$d_\textrm{yz}$ states for interfacial electrons. Moreover, the impact of strain further corroborates that this anomalous orbital structure can be linked to the altered crystal field at the interface and symmetry breaking of the interfacial structural units. Our findings provide another interesting route to engineer emergent quantum states with deterministic orbital symmetry.